We are conducting a randomized, double-blinded controlled trial among pregnant women in Cuernavaca, Mexico, to study the effect of daily supplementation with 200 mg docosahexaenoic acid (DHA) until delivery on a) birth outcomes b) maternal blood and breast milk DMA levels c) postnatal growth and development and d) infant DHA status. This is a collaborative effort among the Department of International Health, Emory University, and the Institute Nacional de Salud Publica in Mexico. The focus of this supplemental study is to evaluate the effect of DHA supplementation on infant immune function. We will analyze infant plasma and maternal breast milk samples currently being collected as part of the parent study protocol. DHA and other n-3 polyunsaturated fatty acids have been shown to modulate immune function in humans; however, few studies have addressed the independent effect of DHA. The central hypothesis of the supplemental study, that maternal DHA supplementation in pregnancy will modulate infant immune function, will be tested by measuring indicators of infant immune function including total immunoglobulin (Ig) concentrations and IgG sub-class concentrations in plasma, to be measured at birth (cord blood) and at 3 months. Antigen-specific antibody response after routine vaccination with 1 dose of diphtheria vaccine will be measured in infants at 3 months. We will measure breast milk IgA concentrations at 1 and 3 months post-partum. The sample size fixed by the parent study will be 116 mother-child pairs per group. The immunological assessments will be done at no cost to the study by the Centers for Disease Control, Atlanta. Data analysis will include group comparisons (intent-to-treat) after ensuring effectiveness of randomization. Adjusted comparisons (using co-variance analysis) will be made if control for potential confounders is required. To our knowledge, there are no studies that have evaluated the impact of maternal DHA supplementation in pregnancy on infant immune function. [unreadable] [unreadable]